HIV-1 reverse transcriptase (RT) is a central target for antiviral treatment of AIDS and detailed knowledge of its structure and function has important clinical and biological consequences. Both nucleoside and non-nucleoside RT inhibitors are used as effective drugs for treating AIDS, but success can be limited by the emergence of drug-resistant viral variants. Novel non-nucleoside RT inhibitors have been developed through structure-based methods, including the recently approved drug etravirine/TMC125/Intelence, and we propose to carry out crystallographic studies that could enable structure-guided improvement of additional classes of RT inhibitors. The proposed work will involve structure determination of wild-type and drug-resistant HIV-1 RT in the presence and absence of nucleic acid substrates and inhibitors. Structural studies of inhibitor will explore binding sites for known drugs (NRTIs and NNRTIs) as well as other potential drug targeting sites including the RNase H active site. Use of strategically modified protein and nucleic acids will enhance the efficiency and quality of the structural studies. The structures determined will enhance our understanding of mechanisms of polymerase and RNase H catalysis, inhibition, and of drug resistance.